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Date: Wed, 13 Mar 2024 13:41:16 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Douglas Anderson <dianders@...omium.org>, Rob Clark <robdclark@...il.com>,
Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC: Bjorn Andersson <andersson@...nel.org>, Daniel Vetter <daniel@...ll.ch>,
David Airlie <airlied@...il.com>, Guenter Roeck <groeck@...omium.org>,
"Kuogee Hsieh" <quic_khsieh@...cinc.com>,
Marijn Suijten
<marijn.suijten@...ainline.org>,
Sean Paul <sean@...rly.run>, Tanmay Shah
<tanmay@...eaurora.org>,
Vinod Polimera <quic_vpolimer@...cinc.com>,
<dri-devel@...ts.freedesktop.org>, <freedreno@...ts.freedesktop.org>,
<linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] drm/msm/dp: Avoid a long timeout for AUX transfer if
nothing connected
On 3/12/2024 5:13 PM, Douglas Anderson wrote:
> As documented in the description of the transfer() function of
> "struct drm_dp_aux", the transfer() function can be called at any time
> regardless of the state of the DP port. Specifically if the kernel has
> the DP AUX character device enabled and userspace accesses
> "/dev/drm_dp_auxN" directly then the AUX transfer function will be
> called regardless of whether a DP device is connected.
>
I do see
"
* Also note that this callback can be called no matter the
* state @dev is in and also no matter what state the panel is
* in. It's expected:
"
I understand about the host state that we need to allow the transfers by
powering on if the host was off.
But I wonder why we should allow the transfer if the sink is not
connected because it will anyway timeout.
Does it make sense to have get_hpd_status() from the aux dev and not
issue the transfers if the sink was not connected?
This is more of questioning the intent of drm_dp_helpers to allow
transfers without checking the sink status.
> For eDP panels we have a special rule where we wait (with a 5 second
> timeout) for HPD to go high. This rule was important before all panels
> drivers were converted to call wait_hpd_asserted() and actually can be
> removed in a future commit.
>
> For external DP devices we never checked for HPD. That means that
> trying to access the DP AUX character device (AKA `hexdump -C
> /dev/drm_dp_auxN`) would very, very slowly timeout. Specifically on my
> system:
> $ time hexdump -C /dev/drm_dp_aux0
> hexdump: /dev/drm_dp_aux0: Connection timed out
>
> real 0m8.200s
>
IIUC, we want to timeout faster by not bailing out if not connected right?
> Let's add a check for HPD to avoid the slow timeout. This matches
> what, for instance, the intel_dp_aux_xfer() function does when it
> calls intel_tc_port_connected_locked(). That call has a document by it
> explaining that it's important to avoid the long timeouts.
>
> Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
>
> drivers/gpu/drm/msm/dp/dp_aux.c | 8 +++++++-
> drivers/gpu/drm/msm/dp/dp_catalog.c | 10 ++++++++++
> drivers/gpu/drm/msm/dp/dp_catalog.h | 1 +
> 3 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_aux.c b/drivers/gpu/drm/msm/dp/dp_aux.c
> index 03f4951c49f4..de0b0eabced9 100644
> --- a/drivers/gpu/drm/msm/dp/dp_aux.c
> +++ b/drivers/gpu/drm/msm/dp/dp_aux.c
> @@ -307,7 +307,8 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux *dp_aux,
> * turned on the panel and then tried to do an AUX transfer. The panel
> * driver has no way of knowing when the panel is ready, so it's up
> * to us to wait. For DP we never get into this situation so let's
> - * avoid ever doing the extra long wait for DP.
> + * avoid ever doing the extra long wait for DP and just query HPD
> + * directly.
> */
> if (aux->is_edp) {
> ret = dp_catalog_aux_wait_for_hpd_connect_state(aux->catalog);
> @@ -315,6 +316,11 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux *dp_aux,
> DRM_DEBUG_DP("Panel not ready for aux transactions\n");
> goto exit;
> }
> + } else {
> + if (!dp_catalog_aux_is_hpd_connected(aux->catalog)) {
> + ret = -ENXIO;
> + goto exit;
> + }
> }
>
> dp_aux_update_offset_and_segment(aux, msg);
> diff --git a/drivers/gpu/drm/msm/dp/dp_catalog.c b/drivers/gpu/drm/msm/dp/dp_catalog.c
> index 5142aeb705a4..93e2d413a1e7 100644
> --- a/drivers/gpu/drm/msm/dp/dp_catalog.c
> +++ b/drivers/gpu/drm/msm/dp/dp_catalog.c
> @@ -266,6 +266,16 @@ int dp_catalog_aux_wait_for_hpd_connect_state(struct dp_catalog *dp_catalog)
> 2000, 500000);
> }
>
> +bool dp_catalog_aux_is_hpd_connected(struct dp_catalog *dp_catalog)
> +{
> + struct dp_catalog_private *catalog = container_of(dp_catalog,
> + struct dp_catalog_private, dp_catalog);
> +
> + /* poll for hpd connected status every 2ms and timeout after 500ms */
> + return readl(catalog->io->dp_controller.aux.base + REG_DP_DP_HPD_INT_STATUS) &
> + DP_DP_HPD_STATE_STATUS_CONNECTED;
> +}
This method of checking HPD status works for devices which use internal
HPD block to control the HPD (like sc7180/sc7280) but not for devices
where HPD is controlled outside the MSM DP controller like sc8280xp,
sc835-/sm8450 etc etc which use pmic_glink and DP driver only receives
the hpd status using the dp_bridge_hpd_notify() callback.
If we want to make this generic, we have to do something like:
dp_hpd_unplug_handle() notifies the dp_aux.c module that status is
disconncted and we should bail out
dp_hpd_plug_handle() notifies dp_aux.c module that status is connected
again and we allow the aux transfers.
> +
> static void dump_regs(void __iomem *base, int len)
> {
> int i;
> diff --git a/drivers/gpu/drm/msm/dp/dp_catalog.h b/drivers/gpu/drm/msm/dp/dp_catalog.h
> index 38786e855b51..1694040c530f 100644
> --- a/drivers/gpu/drm/msm/dp/dp_catalog.h
> +++ b/drivers/gpu/drm/msm/dp/dp_catalog.h
> @@ -86,6 +86,7 @@ void dp_catalog_aux_reset(struct dp_catalog *dp_catalog);
> void dp_catalog_aux_enable(struct dp_catalog *dp_catalog, bool enable);
> void dp_catalog_aux_update_cfg(struct dp_catalog *dp_catalog);
> int dp_catalog_aux_wait_for_hpd_connect_state(struct dp_catalog *dp_catalog);
> +bool dp_catalog_aux_is_hpd_connected(struct dp_catalog *dp_catalog);
> u32 dp_catalog_aux_get_irq(struct dp_catalog *dp_catalog);
>
> /* DP Controller APIs */
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